U.S. patent number 7,860,409 [Application Number 11/842,186] was granted by the patent office on 2010-12-28 for optical receiver, optical receiving apparatus using the optical receiver and optical receiving method thereof.
This patent grant is currently assigned to NEC Corporation. Invention is credited to Tadashi Koga, Morihiko Ota, Masaki Sato.
United States Patent |
7,860,409 |
Sato , et al. |
December 28, 2010 |
Optical receiver, optical receiving apparatus using the optical
receiver and optical receiving method thereof
Abstract
An optical receiver includes an optical amplifier configured to
amplify an input optical signal and output an amplified optical
signal; and a light receiving element configured to convert the
amplified optical signal into an electrical signal and output the
electrical signal, and the optical amplifier controls an
output-level of the amplified optical signal according to a
wavelength of the input optical signal.
Inventors: |
Sato; Masaki (Tokyo,
JP), Ota; Morihiko (Tokyo, JP), Koga;
Tadashi (Tokyo, JP) |
Assignee: |
NEC Corporation (Tokyo,
JP)
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Family
ID: |
39237504 |
Appl.
No.: |
11/842,186 |
Filed: |
August 21, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080212982 A1 |
Sep 4, 2008 |
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Foreign Application Priority Data
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Aug 23, 2006 [JP] |
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2006-225965 |
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Current U.S.
Class: |
398/212;
398/213 |
Current CPC
Class: |
H04B
10/673 (20130101) |
Current International
Class: |
H04B
10/06 (20060101) |
Field of
Search: |
;398/213,212 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Hikari Jyouhou Nettowaku, "Optical Information Network", edited by
Kazuro Kikuchi, Ohmsha, Oct. 2002, p. 169. (on p. 2 of
Specification.). cited by other.
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Primary Examiner: Pascal; Leslie
Attorney, Agent or Firm: Young & Thompson
Claims
What is claimed is:
1. An optical receiver comprising: an optical amplifier configured
to amplify an input optical signal and output an amplified optical
signal; a light receiving element configured to convert said
amplified optical signal into an electrical signal and output said
electrical signal; a memory which records an information to
compensate fluctuation in quantum efficiency on a wavelength of
said light receiving element; and a control unit configured to
control an output-level of said amplified optical signal based on
the information.
2. The optical receiver according to claim 1, wherein said optical
amplifier comprises a control unit controlling said output-level of
said amplified optical signal.
3. An optical receiving apparatus comprising: an optical
demultiplexer configured to demultiplex a wavelength division
multiplexed optical signal into a plurality of optical signals
having mutually different wavelengths and output said plurality of
optical signals; and a plurality of the optical receivers according
to claim 1, each configured to receive a respective one of said
plurality of optical signals.
4. The optical receiver according to claim 1, further comprising: a
managing and monitoring unit configured to provide said wavelength
of said input optical signal for said table.
5. The optical receiver according to claim 1, further comprising: a
channel monitor configured to detect said wavelength from said
input optical signal and provide said wavelength for said
table.
6. The optical receiver according to claim 1, further comprising:
an optical device having wavelength dependency arranged to receive
as an input said amplified optical signal, and to produce as an
output a compensated optical signal, said light receiving element
receiving said compensated optical signal as an input.
7. The optical receiver according to claim 6, wherein said device
having wavelength dependency includes a chromatic dispersion
compensator.
8. An optical receiving method in an optical receiver, the method
comprising: amplifying an input optical signal; outputting the
amplified optical signal; converting the amplified optical signal
into an electrical signal in a light receiving element; outputting
the electrical signal; and controlling an output-level of the
amplified optical signal to compensate fluctuations in quantum
efficiency on a wavelength of the light receiving element.
9. The optical receiving method according to claim 8, wherein the
amplifying step comprising: detecting said wavelength from said
input optical signal; and controlling said output-level of said
amplified optical signal according to said wavelength.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical receiver, an optical
receiving apparatus using the optical receiver and an optical
receiving method thereof, more particularly relates to techniques
to control the power level of an optical signal received at the
optical receiver, the optical receiving apparatus and the optical
receiving method used for wavelength division multiplexing (WDM)
optical transmission systems.
2. Description of the Related Art
An optical receiving apparatus used for conventional wavelength
division multiplexing (WDM) optical transmission systems is
disclosed in reference document 1 (Japanese Patent Application
Laid-open Publication No. 2003-023399, FIG. 29). FIG. 6 shows the
WDM optical transmission system. A wavelength division multiplexed
(WDM) optical signal is demultiplexed by a demultiplexer (DEMUX)
125 and the demultiplexed optical signals are input to each optical
receiver. Each optical receiver has an optical amplifier 127 and an
automatic level control unit (ALC) 128. The ALC 128 controls the
output-level of the optical amplifier 127 by providing feedback
thereto. Therefore, each optical signal has been received by each
receiver 126 at a constant power level. Consequently, even if
wavelength fluctuations happen to an optical signal input to
certain receiver 126, input-level to the receiver 126 is maintained
constant because the output-level from the optical amplifier 127 is
maintained constant.
However, a light receiving element in the receiver 126 generally
has a property that quantum efficiency is different for each
wavelength, so-called "wavelength dependence of quantum
efficiency". The wavelength dependence of quantum efficiency that a
typical light receiving element has is shown in FIG. 7 (refer to
"Hikari Jyouhou Nettowaku (Optical Information Network)" edited by
Kazuro Kikuchi, Ohmsha, October, 2002, p. 169). The horizontal axis
indicates wavelengths and the vertical axis indicates quantum
efficiency. Quantum efficiency of InGaAs/InP, which is mainly used
in WDM optical transmission systems at present, tends to
deteriorate sharply at around 1.6 .mu.m. Therefore, when a
wavelength of light input to the light receiving element varies,
electrical output-level from the light receiving element varies. As
a result, in the reference document 1, when the wavelength varies,
the electrical output-level from the light receiving element varies
even if an optical input-level to the light receiving element is
maintained constant. Until now, the fluctuations that happen in
electrical output-level have been absorbed by an electrical circuit
arranged at a stage after the light receiving element.
As for optical fiber communications in recent years, a speedup is
required with enlarging capacity of information transmission, and
therefore research and development of a 40 Gbps system has been
carried out. The high speed optical transmission system of 40 Gbps
or more places a severe constraint on the dynamic range of an
electrical circuit arranged at a stage after the light receiving
element. For example, in a clock-regeneration and discrimination
circuit as an electrical circuit, the minimum distinguishable
amplitude of the 40 Gbps system is about one order of magnitude
larger than that of a 10 Gbps system. For that reason, the
allowable amount of output amplitude fluctuations in the 40 Gbps
system is about ten times smaller than that in the 10 Gbps system.
Consequently, the 40 Gbps system becomes about ten times weaker
than the 10 Gbps system in amplitude fluctuations of an electrical
signal generated by wavelength fluctuations of an optical signal.
As a result, the 40 Gbps system requires accuracy as high as about
ten times the 10 Gbps system in amplitude adjustments which are
performed according to the wavelength fluctuations.
As mentioned above, with the increase of transmission speed in
recent years, fluctuations in the electrical output-level due to
the wavelength dependence of quantum efficiency cannot be
disregarded, and thereby the fluctuations have become a factor that
makes transmission characteristics worse.
SUMMARY OF THE INVENTION
In view of the foregoing drawbacks of the related art methods and
structures, the present invention seeks to provide an optical
receiver, an optical receiving apparatus and an optical receiving
method that are capable of maintaining an electrical output-level
from a light receiving element constant even when a wavelength of
an optical signal to be input to the light receiving element
varies.
An optical receiver according to the present invention includes an
optical amplifier configured to amplify an input optical signal and
output an amplified optical signal; and a light receiving element
configured to convert the amplified optical signal into an
electrical signal and output the electrical signal, and the optical
amplifier controls an output-level of the amplified optical signal
according to a wavelength of the input optical signal.
An optical receiver according to the present invention includes an
optical amplifier configured to amplify an input optical signal and
output an amplified optical signal; and a light receiving element
configured to convert the amplified optical signal into an
electrical signal and output the electrical signal, and an
output-level of the amplified optical signal is controlled
according to an output-level of the electrical signal.
An optical receiving apparatus according to the present invention
includes an optical demultiplexer configured to demultiplex a
wavelength division multiplexed optical signal into a plurality of
optical signals having mutually different wavelengths and output
the plurality of optical signals; and a plurality of the optical
receivers as described above, each configured to receive a
respective one of the plurality of optical signals.
An optical receiving method according to the present invention
includes amplifying an input optical signal; outputting an
amplified optical signal; converting the amplified optical signal
into an electrical signal; and outputting the electrical signal,
and an output-level of the amplified optical signal is controlled
according to a wavelength of the input optical signal.
An optical receiving method according to the present invention
includes amplifying an input optical signal; outputting an
amplified optical signal; converting the amplified optical signal
into an electrical signal; and outputting the electrical signal,
and an output-level of the amplified optical signal is controlled
according to an output-level of the electrical signal.
Accordingly, with the configuration and method as described above,
the optical receiver, the optical receiving apparatus and the
optical receiving method control the optical output-level from the
optical amplifier, which is arranged at a stage before the light
receiving element, according to the wavelength of the optical
signal to be input to the light receiving element. As a result, the
present invention produces an effect that makes it possible to
maintain the electrical output-level from the light receiving
element constant even when the wavelength of the optical signal to
be input to the light receiving element varies.
BRIEF DESCRIPTION OF THE DRAWINGS
Various aspects, features and advantages of the present invention
will become more apparent from the following detailed description
of preferred embodiments when taken in conjunction with the
accompanying drawings wherein:
FIG. 1 is a block diagram showing an optical receiving apparatus in
a wavelength division multiplexing optical transmission system
according to an embodiment of the present invention;
FIG. 2A is a block diagram showing an optical receiver according to
a first embodiment of the present invention;
FIG. 2B is a block diagram showing another optical receiver
according to a first embodiment of the present invention;
FIG. 3 is a block diagram showing an optical receiver according to
a second embodiment of the present invention;
FIG. 4 is a block diagram showing an optical receiver according to
a third embodiment of the present invention;
FIG. 5 is a block diagram showing an optical receiver according to
a fourth embodiment of the present invention;
FIG. 6 is a block diagram showing an optical receiving apparatus in
a conventional wavelength division multiplexing optical
transmission system; and
FIG. 7 is a diagram showing wavelength dependence of quantum
efficiency of a typical light receiving element.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
An optical receiving apparatus according to a first embodiment of
the present invention, as shown in FIG. 1, includes an optical
demultiplexer 1 and n optical receivers 10-1 to 10-n (n is an
integer of 2 or more) each arranged at respective output-terminals
of the optical demultiplexer 1. The optical receiving apparatus
receives a wavelength division multiplexed (WDM) optical signal in
which n optical signals having mutually different wavelengths,
which are sent from an optical transmitting apparatus (not shown),
are multiplexed. The optical demultiplexer 1 demultiplexes the WDM
optical signal into n optical signals having mutually different
wavelengths and then outputs the demultiplexed n optical
signals.
Each of optical receivers 10-1 to 10-n in FIG. 1, as shown in FIG.
2A, includes an optical amplifier 11, a light receiving element 12,
a lookup table 13, and a post-stage electrical circuit (for
example, amplifier) 14. The optical amplifier 11 amplifies an
optical signal output from the optical demultiplexer 1 and then
outputs the amplified optical signal. The light receiving element
12 converts the optical signal output from the optical amplifier 11
into an electrical signal and then outputs the converted electrical
signal. The lookup table 13 records each wavelength of an optical
signal and an optical output-level of the optical amplifier 11 to
be associated with each other. The post-stage electrical circuit
(for example, amplifier 14) processes the electrical signal output
from the light receiving element 12.
When a wavelength of an optical signal varies, the lookup table 13
transmits information of an optical output-level, which is recorded
to be associated with the wavelength after varying, to the optical
amplifier 11. The optical amplifier 11 receives the information of
the optical output-level at a control unit (not shown) in the
optical amplifier 11. The control unit controls the optical
output-level based on the information. Therefore, fluctuations in
the quantum efficiency of the light receiving element 12 due to the
wavelength fluctuations are compensated, and thereby the electrical
output-level from the light receiving element 12 is maintained
constant.
The lookup table 13 is a table recording correspondence between
each wavelength of the optical signal input to the optical
amplifier 11 and the optical output-level information of the
optical amplifier 11. Therefore, the lookup table 13 enables to
maintain an electric output-level from the light receiving element
12 constant, even when the wavelength of the input optical signal
varies. This table may be created by learning in advance. When a
wavelength of an optical signal input to the light receiving
element 12 varies, the lookup table 13 transmits optical
output-level information corresponding to the wavelength to the
control unit of the optical amplifier 11, according to wavelength
information 14. The control unit controls an electrical
output-level from the optical amplifier 11 to become the optical
output-level associated with the wavelength. Consequently, an
optical input-level to the light receiving element 12 is adjusted
in response to the wavelength. As a result, fluctuations in the
quantum efficiency of the light receiving element 12 generated by
the wavelength fluctuations are compensated, and thereby the
electrical output-level from the light receiving element 12 is
maintained constant.
As one example in which wavelengths of the optical signals input to
the optical receivers 10-1 to 10-n vary, there is a case where an
optical receiver 10-x (x is an arbitrary integer of 1 to n) is
attached to another output-terminal of the optical demultiplexer 1
in place of a certain output-terminal thereof. The low speed
optical transmission system of 10 Gbps or less, as mentioned above,
generally includes an optical amplifier and an automatic level
control unit (ALC). Therefore, since the ALC provides the optical
amplifier with feedback, the optical output-level from the optical
amplifier is controlled. Hence, each receiver can receive the
optical signal at a constant power level, even when the wavelength
of the optical signal varies. As a result, in the case of the low
speed optical transmission system of 10 Gbps or less, amplitude
fluctuations in the electrical signal generated by wavelength
fluctuations can be absorbed by an electrical circuit arranged at a
stage after the light receiving element as long as each receiver
receives the optical signal at a constant power level.
On the other hand, the high speed optical transmission system of 40
Gbps or more, as mentioned above, places a severe constraint on the
dynamic range of an electrical circuit arranged at a stage after
the light receiving element. Therefore, in such a high speed
optical transmission system, the amplitude fluctuations cannot be
absorbed by an electrical circuit arranged at a stage after the
light receiving element, even when each receiver receives the
optical signal at a constant power level by ALC. For that reason,
in the first embodiment, the optical output-level from the optical
amplifier 11 is controlled based on the wavelength information 15
in such a way that an electrical output-level from the light
receiving element 12 becomes a proper value. As a result, the
electrical input-level to an electrical circuit arranged at a stage
after the light receiving element 12 reaches a proper value.
It should be noted that the wavelength information 15 may be
provided from a managing and monitoring device 21 which manages and
monitors the system, as shown in FIG. 2B. When the setting
wavelength of the optical signal is changed, the managing and
monitoring device 21 rewrites the wavelength information 15 based
on the changed information.
In this case, the managing and monitoring device 21 stores
information on the wavelength of an optical signal output from each
output-terminal of the optical demultiplexer 1 beforehand.
Therefore, when the optical receiver 10-x is attached to another
output-terminal of the optical demultiplexer 1, the managing and
monitoring device 21 transmits the wavelength information 15 on the
optical signal output from the output-terminal to the lookup table
13 of the optical receiver 10-x. As a result, the electrical
output-level from the optical amplifier 11 is more properly
compensated based on the wavelength information 15 on the input
optical signal.
In the first embodiment, with the configuration and operation as
described above, the optical output-level from the optical
amplifier, which is arranged at a stage before the light receiving
element, is controlled according to the wavelength of the optical
signal to be input to the light receiving element. Therefore, the
first embodiment produces an effect that makes it possible to
maintain the electrical output-level from the light receiving
element constant, even when the wavelength of the optical signal to
be input to the light receiving element varies. Furthermore, in the
first embodiment, an optical reception-level more suitable for the
light receiving element can be maintained, because the optical
output-level from the optical amplifier is controlled. As a result,
the first embodiment also produces another effect that makes it
possible to ease dynamic range width required for the light
receiving element.
In addition, in the case where the managing and monitoring device
provides the wavelength information, when the setting wavelength of
an optical signal is changed, this case also produces still another
effect that quick level control can be achieved because the
wavelength information is easily gained.
A second embodiment of the present invention will now be described.
As shown in FIG. 3, an optical receiver according to the second
embodiment has a configuration that a channel monitor (wavelength
monitor) 16 is added to the configuration of FIG. 2A according to
the first embodiment. The channel monitor 16 detects a wavelength
of an optical signal input to the optical amplifier 11. The
detected wavelength is transmitted to the wavelength information 15
and then transmitted to the lookup table 13. It should be noted
that a channel monitor 16 may be built in the optical amplifier
11.
With the configuration and operation as described above, the second
embodiment produces effects similar to the first embodiment.
Furthermore, the second embodiment also produces another effect to
make it possible to control the optical output-level from the
optical amplifier in a more timely manner because the wavelength of
the input optical signal is detected.
Next, a third embodiment of the present invention will now be
described. An optical receiver according to the third embodiment
includes a device having wavelength dependency. As shown in FIG. 4,
the optical receiver has a configuration that a chromatic
dispersion compensator 17 as a device having wavelength dependency
is added to the configuration of FIG. 2A according to the first
embodiment. Although the chromatic dispersion compensator 17 has
wavelength dependency for an input optical signal, level
fluctuations in the optical signal due to the wavelength dependency
can also be compensated because the optical output-level from the
optical amplifier is controlled at a constant level as well as the
first embodiment.
With the configuration and operation as described above, the third
embodiment produces effects similar to the first embodiment.
Furthermore, the third embodiment also produces another effect to
make it possible to compensate level fluctuations in the optical
signal due to a device having wavelength dependency.
Next, a fourth embodiment of the present invention will now be
described. As shown in FIG. 5, an optical receiver according to the
third embodiment includes a detector 18 and an automatic level
control circuit (ALC) 19 in place of the lookup table 13 and the
wavelength information 15 in FIG. 2A according to the first
embodiment. The detector 18 detects an electrical output-level of
the light receiving element 12. The automatic level control circuit
19 compares a peak voltage detected by the detector 18 with a
reference voltage 20. And then, the automatic level control circuit
19 controls an optical output-level from the optical amplifier 11
in such a way that the electrical output-level of the light
receiving element 12 becomes constant according to the comparison
result.
In the first through third embodiments, the optical output-level
from the optical amplifier is controlled by using the lookup table
13 where the optical output-level from the optical amplifier is
associated with the respective wavelength of the optical signal. On
the other hand, in the fourth embodiment, the optical output-level
from the optical amplifier is controlled by directly detecting the
electrical output-level from the light receiving element in order
to keep the electrical output-level from the light receiving
element constant.
With the configuration and operation as described above, the fourth
embodiment produces effects similar to the first embodiment. The
fourth embodiment has also another effect that it is not necessary
to make the lookup table beforehand. Furthermore, the fourth
embodiment has also still another effect to make it possible to
control the electric output-level from the light receiving element
more accurately and in a more timely manner because the electric
output-level is directly detected by the detector.
It should be noted that the above embodiments may be used in
combination. For example, in the fourth embodiment shown in FIG. 5,
a device having wavelength dependency such as a chromatic
dispersion compensator may be placed between the optical amplifier
11 and the light receiving element 12 as well as the configuration
of the third embodiment shown in FIG. 4. That case produces an
effect to make it possible to compensate level fluctuations in the
optical signal due to a device having wavelength dependency.
While the present invention has been described in connection with
certain preferred embodiments, it is to be understood that the
subject matter encompassed by way of the present invention is not
to be limited to those specific embodiments. On the contrary, it is
intended for the subject matter of the invention to include all
alternatives, modifications and equivalents as can be included
within the spirit and scope of the following claims.
Further, the inventor's intent is to retain all equivalents of the
claimed invention even if the claims are amended later during
prosecution.
* * * * *